<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<body>
   <p id="p">
       hello world
   </p> 
   <script>
       /*  
       before()和after()是发生在兄弟元素之间
        */
       var p=document.getElementById("p");
       p.before("前面")
    p.after("后面")
   </script>
</body>
</html>